Abstract: Negative correlations between blood viscosity parameters and fitness have been reported, but their physiological meaning remains incompletely understood. Since rheo‐active treatments are used in athletes doping, we aimed at clarifying the relationships between hematocrit (Hct), viscosity and performance by comparing aerobic capacity, overtraining questionnaire, and hemorheological parameters. Subjects and methods: 29 sportsmen (24.71±1.05 yr; 74.90±1.44 kg; 178.5±1.05 cm) underwent a standardised exercise test. Physical working capacity (W170), maximal power output (Wmax ) and maximal oxygen consumption (VO2 max ) were calculated. Viscometric measurements were done with a MT 90 Medicatest viscosimeter. Hct was measured with microcentrifuge. All subjects answered the overtraining questionnaire proposed by the French Society for Sports Medicine. Results: The best correlate of maximal power output (Wmax ) was whole blood viscosity (r=−0.383, p<0.001). The stepwise regression analysis only selected Hct as W170 determinant (r=−0.66, p<0.001). Similarly the best determinant of VO2 max , expressed as a percentage of theoretical values, was Hct (r=−0.462, p=0.01). Hct/viscosity ratio (Hct/η), a proposed index of Hct's positive influence on O2 transfer to tissues, was positively correlated to Wmax expressed as a percentage of theoretical values (r=0.487, p=0.02). The overtraining score was correlated to plasma viscosity (r=0.450, p=0.016). Conclusion: The best hemorheogical correlate of fitness is a low hematocrit and the best hemorheological correlate of overtraining is increased plasma viscosity.